How to build an efficient JavaScript polling mechanism with async functions?

JavaScript polling is a technique for periodically checking on data for updates or changes, as would often be the case when someone is working with APIs or long-running processes. Easy to understand as a concept, setting in place an efficient and robust polling mechanism can be very tricky. This guide will help resolve some common problems associated with JavaScript polling and show how to balance the frequency of polling with resource consumption, correctly handle network errors and timeouts, provide cancellation for long-running polls, prevent excessive API calls, and effectively manage asynchronous operations. It makes use of async functions and modern JavaScript capabilities for a flexible polling solution; therefore, it improves much in performance, handling errors, and control of this process, hence applicable to a wide array of applications, from simple status checks up to complex tasks with data synchronization.

Building a Reliable JavaScript Polling Mechanism

Building a reliable JavaScript polling mechanism with async functions involves setting up a method to fetch data from a server repeatedly until certain conditions are met. This can be very useful in scenarios like tracking the status of a job, keeping real-time data up-to-date, and more. The main goal is to keep resource usage low, handle errors gracefully, and avoid blocking the main thread with tight loops.

So let's dive into how to set this up step by step!

Setting up the Polling Function

The heart of our polling mechanism is an async function that repeatedly calls another async function to grab data. This means using setTimeout or setInterval to wait between fetch attempts.

Here's the basic structure of our polling function:

Function Signature: We'll call our function poll.
Parameters: This function will take in:

fn: The async function that fetches the data.
interval: The waiting period between each poll, in milliseconds.
maxAttempts: (Optional) The maximum number of tries before stopping polling.

Return: A promise that resolves when the condition is met or rejects when an error happens or we reach the max attempts.

Implementing the Polling Function

Here's the polling function all laid out:

async function poll(fn, interval, maxAttempts = Infinity) {
  let attempt = 0;

  async function executePoll(resolve, reject) {
    try {
      attempt++;
      const result = await fn();
      
      // Adjust based on your conditions
      if (result) {
        resolve(result);
      } else if (attempt >= maxAttempts) {
        throw new Error('Max attempts reached');
      } else {
        setTimeout(executePoll, interval, resolve, reject);
      }
    } catch (error) {
      reject(error);
    }
  }

  return new Promise(executePoll);
}

Defining the Async Function to Poll

The async function passed to poll should handle the data-fetching logic. Here's an example where we fetch a job status from an API:

async function fetchJobStatus() {
  try {
    const response = await fetch('/api/job-status');
    
    if (!response.ok) {
      throw new Error('Network response not OK');
    }

    const data = await response.json();

    return data.status === 'completed';

  } catch (error) {
    console.error('Fetch error:', error);
    return false;
  }
}

Using the Polling Function

Now we can use the poll function to check the job status periodically:

(async () => {
  try {
    const result = await poll(fetchJobStatus, 2000, 10); // Poll every 2 seconds, up to 10 times
    console.log('Job completed:', result);
  } catch (error) {
    console.error('Polling failed:', error);
  }
})();

Adding More Robust Features

Custom Condition Check

You can modify the polling function to include an extra conditionFn parameter for custom conditions.

async function poll(fn, interval, conditionFn = result => result, maxAttempts = Infinity) {
  let attempt = 0;

  async function executePoll(resolve, reject) {
    try {
      attempt++;
      const result = await fn();
      
      if (conditionFn(result)) {
        resolve(result);
      } else if (attempt >= maxAttempts) {
        throw new Error('Max attempts reached');
      } else {
        setTimeout(executePoll, interval, resolve, reject);
      }
    } catch (error) {
      reject(error);
    }
  }

  return new Promise(executePoll);
}

You can use this enhanced poll function like this:

(async () => {
  try {
    const result = await poll(
      fetchJobStatus,
      2000,
      status => status === 'completed',  // Custom condition function
      10
    );
    console.log('Job completed:', result);
  } catch (error) {
    console.error('Polling failed:', error);
  }
})();

Exponential Backoff

For cases with rate limiting, we can add exponential backoff:

async function poll(fn, initialInterval, conditionFn = result => result, maxAttempts = Infinity, backoffFactor = 2) {
  let attempt = 0;
  let interval = initialInterval;

  async function executePoll(resolve, reject) {
    try {
      attempt++;
      const result = await fn();
      
      if (conditionFn(result)) {
        resolve(result);
      } else if (attempt >= maxAttempts) {
        throw new Error('Max attempts reached');
      } else {
        setTimeout(executePoll, interval, resolve, reject);
        interval *= backoffFactor; // Exponential backoff
      }
    } catch (error) {
      reject(error);
    }
  }

  return new Promise(executePoll);
}

(async () => {
  try {
    const result = await poll(fetchJobStatus, 1000, status => status === 'completed', 10);
    console.log('Job completed:', result);
  } catch (error) {
    console.error('Polling failed:', error);
  }
})();

With the right intervals, handling errors carefully, and maybe adding exponential backoff, you've got a solid polling mechanism! Tweak those conditions and settings to match your app's needs.

Sources:

Using Fetch - MDN Web Docs

Node.js Timers Documentation

Async Programming - JavaScript Info

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